This invention relates to digital color television camera apparatus and, more particularly, to such apparatus which converts digital primary color image signals generated in accordance with any one of predetermined television standards, such as NTSC or PAL standards, into either a digital component video signal (such as a D-1 video signal) or a digital composite video signal (such as a D-2 video signal).
A typical color television camera includes an image pickup device which produces three primary color image video signals, such as red (R), green (G) and blue (B) signals. A luminance signal (Y) and color difference signals are generated from the R, G and B color video signals in accordance with a particular television standard with which the color video camera is used. Typically, the television standard is the NTSC standard, the PAL standard, the SECAM standard, or other conventional television standards that are used for broadcast purposes throughout the world. Usually, one television camera is used to generate color television signals in the NTSC standard, another is used to generate color television signals in the PAL standard, and so on.
In the NTSC standard, for example, the color difference signals which are generated from the primary color video signals are conventional I and Q color difference signals. In the PAL standard, the color difference signals which are generated from the primary color video signals are conventional U and V color difference signals. With reference to the NTSC standard, the relationship between the luminance signal Y and the color difference signals I and Q is as follows: EQU Y=0.30R+0.59G+0.11 B EQU I=0.60R-0.28G-0.32 B EQU Q=0.21R-0.52G+0.31 B
In the PAL standard, the relationship between the luminance signal Y and the color difference signals U and V is as follows: EQU Y=0.30R+0.59G+0.11B EQU U=B-Y EQU V=R-Y
In the NTSC standard, the I color difference signal is constrained to a band of frequencies and exhibits a prescribed attenuation characteristic (referred to as a frequency band characteristic) such that the attenuation of the I signal at approximately 1.3 MHz is less than 2 dB and the attenuation of the I signal at approximately 3.5 MHz is greater than 20 dB.
The Q color difference signal of the NTSC standard exhibits a narrower bandwidth and a different frequency band characteristic. The attenuation of the Q signal at approximately 0.4 MHz is less than 2 dB, its attenuation at approximately 0.5 MHz is less than 6 dB and its attenuation at approximately 0.6 MHz is greater than 6 dB.
In the PAL standard, the frequency band characteristics of the U and V color difference signals are approximately the same and are constrained such that the attenuation thereof at approximately 1.3 MHz is less than 3 dB and its attenuation at approximately 4 MHz is greater than 20 dB.
In both the NTSC and PAL standards, the luminance signal Y and the color difference signals I and Q (for the NTSC standard) or U and V (for the PAL standard) are produced by combining the primary color video signals R, G and B in a matrix and then band-limiting the resultant color difference signals by means of a low pass filter whose filter characteristics conform to the aforementioned frequency band characteristics.
Typical color television cameras use conventional solid-state imaging systems in which a solid-state image sensor having a discrete pixel structure constituted by charge coupled devices (CCDs) is provided as the imaging section. The solid-state image sensor itself constitutes a sampling system for producing discrete samples of the color video signals R, G and B, which samples can be used to produce either analog or digital video signals. Because of this inherent sampling, aliasing components from the spatial sampling frequency f.sub.s which is used to sample the CCDs are mixed into the image pickup output signals produced by the solid-state image sensor in a predictable manner.
It has been proposed to use a dual type CCD solid-state image sensor for forming the 3-color image. In this dual type CCD sensor, one solid-state image sensor is used to produce the green color image and the other solid-state image sensor is provided with color coding light filters to produce the red and blue color images. It also has been proposed to use three separate CCD image sensors, one for each color. A so-called spatial offsetting technique for improving resolution in the latter arrangement (i.e. in the three CCD image sensor) is known. In this technique, the solid-state image sensor which produces the red color image is offset from the solid-state image sensor which produces the green image by an amount equal to one-half the spatial pixel sampling period; and, likewise, the solid-state image sensor which produces the blue image is offset from the green image sensor by this same one-half spatial pixel sampling period. By using this spatial offsetting technique, a multi-chip solid-state image sensor may be used to produce an analog output signal having high resolution that surpasses the restrictions inherent in the discrete number of pixels provided in the solid-state image sensor.
While a typical color television camera produces analog color video signals at its output, it is desirable to provide a camera whose output signals are in digital form. Such digital video signals should be compatible with the standardized digital recording formats currently in use, such as the so-called 4:2:2 digital component format (referred to by those of ordinary skill in the art as the D-1 format) or the digital composite format (referred to as the D-2 format). In the D-1 format, the luminance signal Y produced by, for example, the usual matrix, is sampled at a sampling frequency 13.5 MHz; and this sampling frequency is the same for both the NTSC standard and the PAL standard. In addition, the sampling frequency of the color difference signals is equal to 6.75 MHz (in both the NTSC and PAL standards). In the D-1 format, the sampled luminance and color difference signals are linearly quantized, and each sample is comprised of 8 bits. Heretofore, typically, the quantization levels of the luminance signal Y for the D-1 format in both the NTSC and PAL standards are such that the black level (0%) is assigned a quantization level of 16 and the white level (100%) is assigned the quantization level 235. In similar manner, the quantization levels of the color difference signals for both the NTSC and PAL standards are such that the quantization level 128 represents the no signal level and the quantization level 225 represents the maximum level, such as in response to a 100% color bar input.
In the D-2 format (which produces digital composite video signals), the luminance sampling frequency f.sub.s is equal to four times the chrominance subcarrier frequency f.sub.sc (f.sub.s =4f.sub.sc) for producing the digital composite video signal in both the NTSC and PAL standards. Of course, in the NTSC standard, f.sub.sc =3.58 MHz and in the
standard f.sub.sc =4.43 MHz. Thus, in the NTSC standard, the digitized video signals exhibit a clock frequency of 14.3 MHz and in the PAL standard, the digitized video signals exhibit a clock frequency of 17.7 MHz. In addition, other sampling and clock frequencies have been adopted for the D-2 format when digitizing color video signals in the PAL standard, namely a clock frequency equal to 908 times the horizontal scanning frequency f.sub.H (f.sub.s =908 f.sub.H) or a clock frequency equal to 944 times the horizontal scanning frequency (f.sub.s =944 f.sub.H) Thus, in the D-2 format, four different sampling clock frequencies have been used:
f.sub.s =4 f.sub.sc =14.3 MHz for NTSC; PA0 f.sub.s =4 f.sub.sc =17.7 MHz for PAL; PA0 f.sub.s =908 f.sub.H for PAL; and PA0 f.sub.s =944 f.sub.H for PAL.
In the D-2 format, the video signal, including the synchronization and burst portions thereof, is linearly quantized; and each quantized sample is represented by 8 bits. In hexadecimal notation, the 8-bit quantization of the video signal for the black level (0%) is 3C and for the white level (100%) is C8. 8-bit quantization of the sync tip level in hexadecimal notation is 04.
One example of color television apparatus which produces digital video signals in accordance with the foregoing sampling clock frequencies and quantization levels is described in U.S. patent application Ser. No. 07/587,066, assigned to the assignee of the present invention.
Although it is desirable to provide a single color television camera capable of producing digital video signals in either the D-1 or D-2 formats for either the NTSC or PAL (or other) standards, certain difficulties in implementing a practical design are evident. For example, different sampling frequencies are used for the D-1 and D-2 formats, and even in the D-2 format, different sampling frequencies are used to digitize video signals in the PAL standard. Also, the quantizing values which are used to represent particular reference levels in the D-1 and D-2 formats, as well as in the NTSC and PAL standards, differ from each other, as mentioned above. Accordingly, it has been thought, heretofore, that different encoder circuits were needed to encode NTSC video signals in the D-1 format, to encode NTSC video signals in the D-2 format, to encode PAL video signals in the D-1 format and to encode PAL video signals in the D-2 format. It will be appreciated that if separate discrete encoder circuits are used to accommodate the D-1 and D- 2 formats for encoding either NTSC or PAL video signals, a complex and expensive color television camera will result. Furthermore, since the frequency band characteristics of the color difference signals in the NTSC standard (for example, the I and Q signals) differ from the frequency band characteristics of the color difference signals in the PAL standard (for example, the U and V signals), the requisite band limiting operation which is used to minimize aliasing effects will require different low pass filter processing for the NTSC and
standards. Even when implemented by digital circuitry, such low pass filter processing is complex and expensive if different digital processing circuits must be used to band-limit the NTSC color difference signals and the PAL color difference signals.
Therefore, it had been thought to be complicated, expensive and, thus, impractical, to design a common color television camera that can be used to digitize both NTSC and PAL color video signals in both the D-1 and D-2 formats. Rather, at best, separate color television cameras were used, depending upon the digital format in which the digitized video signals are encoded and the particular television standard (e.g. NTSC or PAL) with which the digitized video signals are to be used.